Electric filter



Dec. 10, 1940.

T. J. WEYERS ELECTRIC FILTER Filed Feb. 10, 1938 Patented Dec. 10, 1940 PATENT OFFICE- ELEC'I RIC' FILTER Theodorus Josephus Weyers, Eindhoven, Netherlands,-assignor,,by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Commas trustee Application February 10, 1938, Serial No. 189,854 In Germany February 18, 1937 1 Claim. (Cl. 178-44) My inventionrelates to-electric filters.

In designing electric filters one frequently starts from the ideal case in which a filter is terminated at its input and output terminals attenuation curve of such a practical filter differs from curve a, and has, for example, a shape similar to the dotted curve indicated by reference letter b. As appears from Fig. 1, the attenu- 5 by image impedance and comprises coils and ation of curve b in the pass-band is not constant 5 condensers which cause no losses. Such an ideal but varies to an appreciable amount with the filter has two cut-off frequencies f1 and f2, both frequency. This phenomenon is very undesirof which have finite values in the case of a bandable and is eliminated or at least greatly diminpass filter, and one of which has an infinite or ished by the present invention.

zero value inthe case of a high-pass or low-pass Fig. 2 schematically illustrates a filter F to 10 filter. Such an ideal filter has no attenuation which is connected an output impedance Zb. In in the pass-band, e. g. in the region between the accordance with the invention, a series resotwo out off frequencies, but frequencies lying nance circuit comprising an inductance coil L, outside this frequency band are attenuated. a condenser C, and a resistance R is connected However, in practice the attenuation in the passacross the output impedance. If L and C are 15 band is not constant but varies to a large extent given such values that series-resonance occurs with the frequency because the condensers and at the mean frequency fm of the transmitted frecoils have losses and the filter is not terminated quencies, the circuit LCR behaves for this freby image impedances. quency as if only the resistance R- were present.

The object of my invention is to overcome As resistance R is connected in parallel with the 20 the above difiiculties and to provide a filter whose output impedance Zb, it therefore increases the attenuation curve is substantially fiat in the passattenuation of the filter, and by giving it a proper band. value it is possible to give, the attenuation for According to the invention I connect in series the mean frequency in a value (see Fig. 1) suband/or in parallel with a terminal impedance stantially equal to or preferably slightly larger 25 and/or between two filter sections, a resonance than the attenuation of the filter at the out off circuit tuned to a frequency lying in the passfrequencies f1 and f2 without the resonance cirband and comprising an ohmic resistance, the incuit. Although for frequencies higher or lower ductance and capacity of the resonance circuit than the mean frequency to which L and C are and the resistance being so selected that the attuned, the impedance of circuit LCR increases 30 tenuation in the pass-band is substantially conand brings about attenuation, this is less than stant. for the mean frequency fm. In fact, as appears In order that the invention may be more clearfrom Fig. l, circuit LCR has only a slight influly understood and readily carried into effect, I ence at the cut off frequencies f1 and i2, and at shall describe the same in more detail with these points brings about substantially the same 35 reference to the accompanying drawing in which: attenuation as in the absence of the circuit.

Figure l is a graph for use in explaining the Thus an attenuation curve having a substantially invention; and fiat shape in the pass-band can be obtained by Figs. 2 and 3 are schematic diagrams of filters giving the attenuation for the ,mean frequency according to the invention. a value which is substantially equal to that of 40 In the graph of Fig. l the frequency (f) and the attenuation for the out off frequencies. the attenuation (u) ,are plotted as abscissae and The circuit arrangement schematically illusthe ordinates respectively, and reference letter a trated in Figure 3 is the same as that of Fig. 2, indicates the damping curve of an ideal bandexcept that a parallel resonance circuit LCR is pass filter. The cut off frequencies of the passconnected in series with the terminal impedance 45 band are denoted by f1 and f2. Zb, instead of a series resonance circuit con- However, as stated above, the coils and connected in parallel with the terminal impedance densers of an actual filter have losses, and in as shown in Figure 2. Such a circuit-arrangepractice the condition that the filter should be ment gives the same results as that of Fig. 2. It

terminated by impedances which are, for all the is also possible to connect the series resonance 50 frequencies, equal to the ima e impedances Which circuit of Fig. 2 or the parallel resonance circuit depend on frequency is SeldOm fulfilled. The of Fig. 2 between two filter sections or into the terminal impedances generally are, for example, input circuit of the filter. ohmic resistances which are so selected that, for In the above it was assumed that the filters the mean frequency had, before the application of the resonance cir- 55 f cuit, a symmetrical attenuation curve so that f1 f2 the resonance circuit was tuned to the mean freof the transmitted frequencies, the terminal requency. In those cases in which the attenuation sistances are equal to the image impedances. curve is unsymmetrical with respect to the mean frequency, it is, however, more advantageous to 0 Due to the losses and to the mismatching, the

tune the resonance circuit to a frequency which differs from the mean frequency, for example to that frequency in the pass-band for which the attenuation is smallest.

The invention may also be applied to highpass and low-pass filters which can be considered as special types of band-pass filters in which fm=fz= and fm=f1=0 respectively, and also to band-eliminating filters.

While I have described my invention inconnection with specific examples and applications, I do not Wish to be limited thereto, but desire the appended claim to be construed as broadly as permissible in view of the prior art.

What I claim is:

A circuit arrangement comprising an electric filter, an impedance connected to the terminals of said filter, and means for producing a substantially constant attenuation in the pass-band of the filter comprising a resonance circuit tuned to a frequency lying within the pass-band of the THEODORUS JOSEPHUS 'WEYERS. 

